The distinguishing characteristic of saline soils from the agricultural standpoint, is that they contain sufficient neutral soluble salts to adversely affect the growth of most crop plants. For purposes of definition, saline soils are those which have an electrical conductivity of the saturation soil extract of more than 4 dS/m at 25°C (Richards 1954). This value is generally used the world over although the terminology committee of the Soil Science Society of America has lowered the boundary between saline and non-saline soils to 2 dS/m in the saturation extract. Soluble salts most commonly present are the chlorides and sulphates of sodium, calcium and magnesium. Nitrates may be present in appreciable quantities only rarely. Sodium and chloride are by far the most dominant ions, particularly in highly saline soils, although calcium and magnesium are usually present in sufficient quantities to meet the nutritional needs of crops. Many saline soils contain appreciable quantities of gypsum (CaSO 4 , 2H 2 O) in the profile. Soluble carbonates are always absent. The pH value of the saturated soil paste is always less than 8.2 and more often near neutrality (Abrol et al., 1980). Physico-chemical characteristics in respect of a few typical saline soil profiles are presented in Tables 5-8.
Excess salts keep the clay in saline soils in a flocculated state so that these soils generally have good physical properties. Structure is generally good and tillage characteristics and permeability to water are even better than those of non-saline soils. However, when leached with a low salt water, some saline soils tend to disperse resulting in low permeability to water and air, particularly when the soils are heavy clays. Leaching may also result in a slight increase in soil pH due to lowering of salt concentration but saline soils, as will be shown later, rarely become strongly sodic upon leaching if there is an adequate drainage system.
In field conditions, saline soils can be recognized by the spotty growth of crops and often by the presence of white salt crusts on the surface. When the salt problem is only mild, growing plants often have a blue-green tinge. Barren spots and stunted plants may appear in cereal or forage crops growing on saline areas. The extent and frequency of bare spots is often an indication of the concentration of salts in the soil. If the salinity level is not sufficiently high to cause barren spots, the crop appearance may be irregular in vegetative vigour.
Moderate salinity, however, particularly if it tends to be uniform throughout the field, can often go undetected because it causes no apparent injuries other than restricted growth. Leaves of plants growing in salt infested areas may be smaller and darker blue-green in colour than the normal leaves. Increased succulence often results from salinity, particularly if the concentration of chloride ions in the soil solution is high. Plants in salt-affected soils often have the same appearance as plants growing under moisture stress (drought) conditions although the wilting of plants is far less prevalent because the osmotic potential of the soil solution usually changes gradually and plants adjust their internal salt content sufficiently to maintain turgor and avoid wilting.
Symptoms of specific element toxicities, such as marginal or tip burn of leaves, occur as a rule only in woody plants. Chloride and sodium ions and boron are the elements most usually associated with toxic symptoms. Non-woody species may often accumulate as much or more of these elements in their leaves without showing apparent damage as do the woody species.
* pH S - pH measured on soil saturated paste.Table 6 TYPICAL SALINE SOIL REPRESENTING ADDALA SERIES, IRAQ (Sehgal, 1980)
Depth cm
Mechanical Composition %
ECe dS/m
Composition of the Saturation Extract me/l
Organic Matter %
Clay
Silt (2-50 m )
Sand (50 m - 2 mm)
SO 4 -
0 - 15
15 - 37
37 - 66
66 - 127
127 - 136
Depth cm
Mechanical Composition %
ECe dS/m
Composition of the Saturation Extract me/l
Organic Matter %
Clay
Silt (2-50 m )
Sand (50 m - 2 mm)
SO 4 -
0 - 17
17 - 57
57 - 85
85 - 108
108 - 123
Depth cm
Mechanical Composition %
ECe dS/m
Composition of the Saturation Extract me/l
Organic Matter %
Clay
Silt (2-50 m )
Sand (50 m - 2 mm)
SO 4 -
0 - 17
1 140
17 - 48
48 - 75
75 - 128
128 -150
The effect of dissolved salts on plant growth depends on their concentration in the soil solution at any particular time but it is extremely difficult to measure the soil solution concentration at the usual field moisture contents due to sampling problems. A simplified procedure consists of mixing a soil sample with sufficient water to produce a saturated paste and then extracting the solution for measurement of conductivity. Measuring the electrical conductivity (EC) of a saturation extract has an advantage in that saturation percentage is directly related to field moisture range. Over a considerable textural range, saturation percentage is approximately four times the moisture content held at fifteen atmospheres which closely approximates the wilting percentage. The soluble salt concentration in the saturation extract therefore tends to be about one-half of the concentration of the soil solution at the upper end of the field available moisture range and about one-fourth the concentration that the soil solution will have at the dry end of the available moisture range (Richards, 1954).
The standard unit of conductance is siemens (see Table 9) and when expressed per unit of distance, the standard unit of conductivity is siemens per metre. The conductivity of most saturation paste extracts is only a fraction of a siemens per metre. For convenience, therefore, conductivities of soil extracts are expressed in deci Siemens (mS) per metre at 25°C.
EC measurements are quick and sufficiently accurate for most purposes. To determine EC the solution is placed between two electrodes of constant geometry and constant distance of separation. When an electrical potential is imposed, the amount of current varies directly with the total concentration of dissolved salts. At constant potential, the current is inversely proportional to the solutions resistance and can be measured with a resistance bridge. Conductance is the reciprocal of resistance and has the unit Siemens (formerly, mhos).
The measured conductance is the result of the solutions salt concentration and the electrode geometry. The effects of electrode geometry are embodied in the cell constant and this is related to the distance between electrodes divided by their effective cross sectional area. The cell constant is commonly obtained by calibration with KCl solutions of known concentration. The conductivity of standard KCl solutions is available in handbooks.
Several empirical relationships have been developed for converting one type of analysis to another. Some of these, summarized in Table 9, are useful for routine verification of data consistency.
The four electrode technique for measuring bulk soil electrical conductivity has been developed (Rhoades, 1976) for use on irrigated soils and on dry land saline seeps in the field. This is a relatively new technique that has great potential for measuring soil salinity in the field without soil sampling and subsequent laboratory analysis. The technique can be used to great advantage for diagnosis and monitoring salinity changes due to season or cultural practices including cropping, etc.
Excess soil salinity causes poor and spotty stands of crops, uneven and stunted growth and poor yields, the extent depending on the degree of salinity. The primary effect of excess salinity is that it renders less water available to plants although some is still present in the root zone. This is because the osmotic pressure of the soil solution increases as the salt concentration increases. Apart from the osmotic effect of salts in the soil solution, excessive concentration and absorption of individual ions may prove toxic to the plants and/or may retard the absorption of other essential plant nutrients.
Table 9 SOME USEFUL CONVERSION FACTORS
Note: The SI unit of conductivity is Siemens symbol S per metre. The equivalent non-SI unit is mho and 1 mho = 1 Siemens. Thus for those unused to the SI system mmhos/cm can be read for dS/m without any numerical change.
Conductivity 1 S cm -1 (1 mho/cm) = 1000 mS/cm (1000 mmhos/cm)
1 mS/cm -1 (1 mmho/cm) = 1 dS/m = 1000 mS/cm (1000 micromhos/cm)Conductivity to mmol (+) per litre:
mmol (+)/1 = 10 × EC (EC in dS/m)for irrigation water and soil extracts in the range 0.1-5 dS/m.
Conductivity to osmotic pressure in bars:
OP = 0.36 × EC (EC in dS/m)for soil extracts in the range of 3-30 dS/m.
Conductivity to mg/l:
mg/l = 0.64 × EC x 10 3 , or (EC in dS/m)
mg/l = 640 × EC
for waters and soil extracts having conductivity up to 5 dS/m.
nmol/l (chemical analysis) to mg/l:
Multiply mmol/l for each ion by its molar weight and obtain the sum.
There is no critical point of salinity where plants fail to grow. As the salinity increases growth decreases until plants become chlorotic and die. Plants differ widely in their ability to tolerate salts in the soil. Salt tolerance ratings of plants are based on yield reduction on salt-affected soils when compared with yields on similar non-saline soils. Soil salinity classes generally recognized are given in Table 10.
Table 10 SOIL SALINITY CLASSES AND CROP GROWTH
Soil Salinity Class
Conductivity of the Saturation Extract (dS/m)
Effect on Crop Plants
Non saline
0 - 2
Salinity effects negligible
Slightly saline
2 - 4
Yields of sensitive crops may be restricted
Moderately saline
4 - 8
Yields of many crops are restricted
Strongly saline
8 - 16
Only tolerant crops yield satisfactorily
Very strongly saline
Only a few very tolerant crops yield satisfactorily
The amount of crop yield reduction depends on such factors as crop growth, the salt content of the soil, climatic conditions, etc. In extreme cases where the concentration of salts in the root zone is very high, crop growth may be entirely prevented. To improve crop growth in such soils the excess salts must be removed from the root zone. The term reclamation of saline soils refers to the methods used to remove soluble salts from the root zone. Methods commonly adopted or proposed to accomplish this include the following:
Scraping: Removing the salts that have accumulated on the soil surface by mechanical means has had only a limited success although many farmers have resorted to this procedure. Although this method might temporarily improve crop growth, the ultimate disposal of salts still poses a major problem.i. Quantity of water for leachingFlushing: Washing away the surface accumulated salts by flushing water over the surface is sometimes used to desalinize soils having surface salt crusts. Because the amount of salts that can be flushed from a soil is rather small, this method does not have much practical significance.
Leaching: This is by far the most effective procedure for removing salts from the root zone of soils. Leaching is most often accomplished by ponding fresh water on the soil surface and allowing it to infiltrate. Leaching is effective when the salty drainage water is discharged through subsurface drains that carry the leached salts out of the area under reclamation. Leaching may reduce salinity levels in the absence of artificial drains when there is sufficient natural drainage, i.e. the ponded water drains without raising the water table. Leaching should preferably be done when the soil moisture content is low and the groundwater table is deep. Leaching during the summer months is, as a rule, less effective because large quantities of water are lost by evaporation. The actual choice will however depend on the availability of water and other considerations. In some parts of India for example, leaching is best accomplished during the summer months because this is the time when the water table is deepest and the soil is dry. This is also the only time when large quantities of fresh water can be diverted for reclamation purposes.
It is important to have a reliable estimate of the quantity of water required to accomplish salt leaching. The initial salt content of the soil, desired level of soil salinity after leaching, depth to which reclamation is desired and soil characteristics are major factors that determine the amount of water needed for reclamation. A useful rule of thumb is that a unit depth of water will remove nearly 80 percent of salts from a unit soil depth. Thus 30 cm water passing through the soil will remove approximately 80 percent of the salts present in the upper 30 cm of soil. Similarly, to reduce the salt content of the surface 60 cm of soil to about 20 percent of the original value would require the passage of about 60 cm of water through the soil. For more reliable estimates, however, it is desirable to conduct salt leaching tests on a limited area and prepare leaching curves. Leaching curves (Figure 1) relate the ratio of actual salt content to initial salt content in the soil (Sa/Sb) to the depth of leaching water per unit depth of soil (Dw/Ds). Results of leaching tests on three soils in Iraq (Dieleman, 1963) presented in Figure 1 show the effect of soil type on the quantity of water required to achieve the same extent of leaching. Results of one such test (Khosla et al., 1979) are presented in Figure 2 (a and b) and some of the soil characteristics of the experimental site are given in Table 11. Figure 2a shows the actual salt distribution following the passage of different quantities of water while Figure 2b relates the depth of water per unit soil depth to the fraction of salts leached (leaching curve). Information on these aspects is important when planning reclamation of large areas.
Figure 1 Typical leaching curves for soils in Iraq (Dieleman, 1963)
Figure 2a Effect of passage of different quantities of water on salt distribution (Khosla et al., 1979)
Figure 2b The leaching curve using data from Figure 2a. Subscript O indicates leaching, eq represents equilibrium value of electrical conductivity in existing irrigated conditions
Table 11 SOIL CHARACTERISTICS OF THE LEACHING SITE (Khosla et al., 1979)
Depth cm
Texture
Bulk Density
ECe dS/m
pH S
Saturation Extract Composition me/I
(g/cm 3 )
(Ca+Mg) 2+
125.7
15-30
30-45
45-60
60-75
75-90
* SL = Sandy Loamii. Water application method
Results from several laboratory experiments (Miller et al ., 1965; and Nielsen and Biggar, 1961) and some field trials (Biggar and Nielsen, 1962; Nielsen et al ., 1966; and Oster et al ., 1972) have shown that the quantity of salts removed per unit quantity of water leached can be increased appreciably by leaching at soil moisture contents of less than saturation, i.e. under unsaturated conditions. In the field unsaturated conditions during leaching were obtained by adopting intermittent ponding or by intermittent sprinkling at rates less than the infiltration rate of the soil. Nielsen et al . (1966) for example, showed that 25 cm of sprinkled water reduced the salinity of the upper 60 cm of soil to the same degree as 75 cm of ponded water.
Figure 3 Effect of method of irrigation and water redistribution following irrigation 2 and evaporation on the salt concentration 3 profiles (Bresler and Hanks, 1969) (flooding)
Figure 3 Effect of method of irrigation and water redistribution following irrigation 2 and evaporation on the salt concentration 3 profiles (Bresler and Hanks, 1969) (sprinkler)
The salt concentration profiles in a flooded and a sprinkler irrigated soil are demonstrated in Figure 3. In both irrigation methods, at the end of irrigation, upper parts of the soil profiles have low concentration of salts and these will depend on the salt concentration of the applied irrigation water. The salt in the profile increases to a maximum value close to the wetting front and drops to its initial value below the wetting depth. Because of a slower wetting rate under sprinkling, the zone of complete leaching at the end of irrigation extends more deeply into the profile than under flood irrigation. When the soil is subjected to evaporation, water carrying salts moves simultaneously in the upward and downward directions. Thus some salts continue to move down with the redistributed water and, at the same time, salts near the surface move towards the soil surface where they accumulate. The amount of salts which move to the surface depend on the amount of salts present in the upper soil layers from where the water can flow upwards. Thus only a small fraction of salts move up during evaporation from the soil previously irrigated by sprinklers. In flooded soils, on the other hand, more salts move upward and accumulate in the soil surface.
Whether an amendment (e.g. gypsum) is necessary or not for the reclamation of salt-affected soils is a matter of practical importance. Saline soils are dominated by neutral soluble salts and at high salinities sodium chloride is most often the dominant salt although calcium and magnesium are present in sufficient amounts to meet the plant growth needs. Since sodium chloride is most often the dominant soluble salt, the SAR of the soil solution of saline soils is also high (Table 11). Figure 4 shows the effect of leaching such a soil with a low electrolyte (EC 0.25 dS/m) water on the resulting SAR of soil solution. The data in this Figure demonstrate that an increasing passage of water resulted in desalinization and simultaneous desodication, i.e. reduction in soil solution SAR although, compared to desalinization, a somewhat greater quantity of water was required to attain the same degree of desodication. When a soil solution is diluted by a factor X, the reduced ratio
will decrease by a factor . This implies that desodication will always accompany desalinization. A favourable calcium to sodium ratio of the irrigation water and any supply of inherent calcium from the soil is likely to further accelerate the desodication process.
Figure 5 shows the distribution of salts and SAR changes when leached with, and without, application of gypsum. It is seen that the salt displacement was about the same in the two treatments, the depth of water applied being 35 cm. While the upper soil layers had nearly the same resultant SAR after leaching, the SAR of the soil solution of deeper soil layers was somewhat lower in the case of gypsum treatment. But since the desalinization and desodication processes proceed simultaneously, it is expected that the SAR of the profile resulting from leaching with gypsum could also be achieved with leaching alone if more water was passed through the soil. The desodication curve (Figure 4) indicates that with an additional 16 cm of leaching water, the SAR of the soil profile could be reduced to the level of the SAR of gypsum treated soil.
Dieleman (1963) and Leffelaar and Sharma (1977) also reported that an amendment may not be needed for reclamation of saline soils having high SAR. The effect of gypsum application on the infiltration rate of a saline soil upon leaching shows in Figure 6 a higher cumulative intake when gypsum was applied. These studies indicate that the application of an amendment, per se , might not be essential for either desalinization or desodication but could hasten the process by maintaining a higher infiltration rate by continuously supplying soluble calcium to the leaching water. Thus, the decision to use an amendment for the reclamation of saline soils having excess neutral soluble salts and a high SAR of soil solution (the so called saline-sodic soils) would depend on soil infiltration characteristics and the electrolyte level of the irrigation water. Light textured soils and those having a favourable infiltration rate are not likely to respond to gypsum application. In heavy textured soils, and where such soils are leached with low electrolyte water, application of an amendment is desirable to hasten reclamation. When any large-scale reclamation is undertaken, the need for application of amendments and their quantities must be established by trials on an experimental scale.
Figure 4 Changes in SAR in relation to depth of leaching water per unit depth of soil. Subscript O indicates before leaching and eq represents equilibrium value of SAR under existing soil-irrigation water conditions (Khosla et al., 1979)
Figure 6 Effect of gypsum application on cumulative infiltration in a saline soil. a - with, b - without gypsum.
Irrigation is the most effective means of stabilizing agricultural production in areas where the rainfall is either inadequate for meeting the crop requirements or the distribution is erratic. Before the introduction to an area of large quantities of water through irrigation, there exists a water balance between the rainfall on the one hand and stream flow, groundwater table, evaporation and transpiration on the other. This balance is serously disturbed when additional quantities of water are artificially spread on the land to grow agricultural crops, introducing (Plates 3, 4a, 4b) additional factors of groundwater recharge from seepage from canals, distributors and field channels, most of which are unlined, and from the irrigation water let on to the fields over and above the quantities actually utilized by the crops, etc. As a result of these, the groundwater table rises. There are numerous instances throughout the world, where consequent upon the introduction of canal irrigation, the water table has risen considerably within 10 years to less than 2 m. Once the groundwater table is close to the soil surface, due to evaporation from the surface, appreciable movement of the groundwater takes place resulting in the accumulation of salts in the root zone. A schematic relationship between depth of groundwater and evaporation from the soil surface is shown in Figure 7. This relationship is significant and shows that there is a critical depth of water table above which there is a sharp increase in the evaporation rate and therefore soil salinization. In general, the critical depth of water table ranges between 1.5 to 3.0 metres depending on soil characteristics, root zone of crops, salt content of groundwater, etc: To ensure a salt-free root zone, evaporation from the groundwater must be prevented thus keeping the groundwater table below the depth that will cause rapid soil salinization. Provision of adequate drainage measures is the only way to control the groundwater table. Subsurface drainage problems may also arise due to the presence, at some soil depth, of a clay barrier, a hardpan, bed rock, or even a subsoil textural change.
In many areas drainage problems also arise because of the accumulation and stagnation of rainfall or excess irrigation water on the soil surface. Surface drainage problems usually arise due to slopes that are too flat or to slow water penetration because of structural instability of the soils or to uneven land. Temporary water stagnation in standing crops results in problems of aeration, disease, weed control and nutrient supply. Proper land shaping and provision of surface drains are needed to solve the problems of surface water stagnation. The experience of some countries in tackling drainage problems and the nature and properties of various drainage materials are described in two FAO publications (FAO, 1971a; 1972).
Figure 7 A schematic relationship between the depth of groundwater and relative evaporation rate from soil surface
i. Surface drainageIn surface drainage, ditches are provided so that excess water will run off before it enters the soil. However the water intake rates of soils should be kept as high as possible so that water which could be stored will not be drained off. Field ditches empty into collecting ditches built to follow a natural water course. A natural grade or fall is needed to carry the water away from the area to be drained. The location of areas needing surface drainage can be determined by observing where water is standing on the ground after heavy rain. Field ditches and collection or outlet ditches should be large enough to remove at least 5 cm of water in 24 hours from a level to a gently sloping land. The capacity of a drainage system should be based on the amount and frequency of heavy rains. How quickly water runs into ditches depends on the rate of rainfall, land slope and the condition of the soil surface including the plant cover. The area that a ditch can satisfactorily drain depends on how quickly water runs into the ditch, the size of the ditch, its grade or slope and its irregularity. The latter is measured by the roughness and the contents of debris and growing vegetation in the ditch. In relatively level areas (slope < 0.2%) a collecting ditch may be installed along one side and shallow v-shaped field ditches constructed to discharge into this collecting ditch. Field ditches used to discharge water into collecting ditches should be laid out parallel to each other 20 to 60 m apart. They should be 30 to 45 cm deep depending upon the depth of the collecting ditch. Care should be taken to avoid sharp curves in the ditches to lessen erosion of the banks. Before planning a detailed surface drainage of an area a standard handbook on the subject should be consulted (for example, ILACO, 1981).
ii. Subsurface drainage
If the natural subsurface drainage is insufficient to carry the excess water and dissolved salts away from an area without the groundwater table rising to a point where root aeration is affected adversely and the groundwater contributes appreciably to soil salinization, it may be necessary to install an artificial drainage system for the control of the groundwater table at a specified safe depth. The principal types of drainage systems may consist of horizontal relief drains such as open ditches, buried tiles or perforated pipes or in some cases pumped drainage wells (Plate 5). a. Open ditches: Open drainage ditches are advantageous for removing large volumes of either surface or subsoil water from land and for use where the water table is near the surface and the slope is too slight for proper installation of tile drains. Where subsurface tile drains are uneconomic or physically impossible, as in many heavy clay soils and where the topography is nearly flat, open drains may be the only practical means of draining the land. Open ditches also serve as outlets for tile drains where their depth is sufficient and other conditions are favourable. The chief disadvantage of open drains is that they occupy land that might otherwise be put to cultivation; open ditches across cultivated fields also obstruct farming operations and are a danger to the livestock and are more costly to maintain than the subsurface covered drains. Open drains become ineffective due to growth of weeds, collapse of banks resulting in partial filling with soil material, etc., and must be periodically cleaned.
b. Mole drains: these are channels left by a bullet shaped device pulled through the soil, they have been used successfully for shallow subsurface drainage of heavy clay soils in many, relatively humid, parts of Europe but have been found impractical with soils of coarser texture. Mole drains are generally cheaper to install than tile or plastic tubings but may last only for two or three years. In addition to being temporary, mole drains are generally shallow and have not been used extensively where salinity build up from the groundwater table is a major problem.
c. Other subsurface drains: These include any type of buried conduit with open joints or perforations that collect and convey excess water from the soil. The conduits may be made from clay, concrete, plastic or other synthetic material but clay and concrete tiles have been the most widely used. Clay tiles are generally manufactured in 30 and 60 cm lengths and have an inside diameter of 10 to 25 cm. They are made from surface clay or shale, which is pulverized, extruded through a die, dried and then burnt in a kiln. Clay tiles are not affected by acid or sodic soils but those made from surface clay or poorly burnt tiles are subject to deterioration by freezing and thawing action. Good quality clay tiles have been found to last indefinitely in the soil. Concrete tiles are made from sand and gravel aggregate and steam or water-cured to obtain the desired strength. Concrete tiles are resistant to freezing and thawing but may be subject to deterioration in acid and sodic soils. For such soils the tiles should be made with cement having a special chemical composition. Water enters the tiles at the butt joints or spaces between adjacent sections. Both clay and concrete tiles may have fitted ends and be perforated for easier entry of water. All drain tiles should meet standard specifications.
Since the nineteen sixties, thermoplastic tubing has become a common drain material. High density polyethylene and polyvinylchloride are the two most common materials. The plastic tubing is corrugated and, unlike clay or concrete tiles, flexible and will deflect vertically when soil is backfilled in the trench. As it deflects, the sides of the tubing move outward horizontally into the surrounding soil. The circular tubing changes to a slight oval shape, which becomes stabilized because the soil on the sides of the tubing resists the further outward movement. Corrugations in the tubing provide sufficient stiffness to resist the initial soil load. They also reduce the amount of plastic required to make the tubing as well as provide flexibility which permits the smaller size tubing to be coiled into a compact package. Plastic drain pipes are generally available in 8 to 30 cm diameters and usually come in rolls of 75 to 80 m long depending on the diameter of the tubing. Corrugated plastic tubing weighs only 1/25 of a concrete or clay tile, resists practically all soil chemicals and can be installed in continuous lengths. Compared to concrete and clay tiles, greater care is required in placing soil around plastic- tubing because of deflection. Water enters the drain tube through sawed slits or cut holes spaced uniformly around and along the tube.
iii. Filter materialsThese are sometimes placed around subsurface drains primarily to prevent the inflow of soil into the drains which may cause failure, and/or to increase the effective diameter or area of openings in the drains which increases inflow rate. Two types of materials are generally used: - thin sheets such as of fibre glass or spun nylon, and
- sand and gravel envelopes or other porous granular materials.
The thin sheet filters may be sealed on to the plastic tubing at the manufacturing site or they may be installed above and/or below the drains as they are being laid. Granular materials should be placed above or below the drains during installation. Such materials must have the proper gradation of sizes to prevent the inflow of soil.
The principal factors affecting the costs involved in installing subsurface drainage systems in large areas are the spacing and depth of drains. Many mathematical equations have been developed to arrive at the optimum depth and spacing for drains but in practice these have found limited application because of the difficulty and high cost of obtaining soil hydraulic conductivity data and related soil and crop interactions. For these reasons the depth and spacing of drains are based largely on experience and judgement (Schwab et al ., 1966).
Subsurface tile or plastic drains are relatively permanent when correctly installed and protected. Large-scale subsurface drainage systems have been in operation in the western United States for nearly fifty years. Extensive installations for water table and salinity control are now being made in many countries including Iraq, Egypt, Australia, etc.
iv. Pump drainage
The chief drawback of gravity drainage systems is their inability to lower the water table to an adequate depth. Pumping groundwater in areas where a suitable permanent aquifer exists is often an effective means of lowering the water table. A decision to pump groundwater for drainage is generally favoured by adequate depths and permeabilities of the water bearing formations, by high values of pumped water for irrigation and by low power costs (see section 4.8.1 on drainage of sodic soils). To determine if pumping would be effective, pumping tests have to be carried out in test wells to determine the feasibility and area of influence by measuring water levels in adjacent observation wells or piezometers. Spacing, depth, and capacity of the pumped wells and other operational details also need to be evaluated from these tests.
v. Maintenance of drainage systems
After a subsurface drainage system has been installed, a suitable map should be made and filed with the property deed. The map should show the location of all ditches and subsurface drains, tile size and grade, depth and spacing. Any subsequent changes should also be recorded on the map. The record is of considerable value to the present and future land owners when the drainage system might need repairs or maintenance.
A subsurface drainage system normally requires little maintenance if it is properly designed and installed. The outlet ditch should be kept free of the sediment and the tile outlet should be protected against erosion and undermining.
If a drain line becomes filled with sediment or roots the line should be uncovered at some point downstream to locate the obstruction. If the line is not completely clogged and water is available the sediment can sometimes be flushed out. A suitable plug, swab or a rigid rod can be used to remove the blockage. A high pressure water jet may be needed to clean out some lines. Often it is more economic to replace the entire plugged section.
Roots of nearby trees can also block subsurface drains. For this reason shrubs and trees growing adjacent to a tile line should be removed. If the tile lines become filled with roots, it is best to dig up and replace the clogged section and remove the troublesome trees at the same time.
The maintenance of open collecting ditches is most important and it is difficult. Weed growth must be controlled and the caving in of the sides requires continuous attention in order that the entire drainage system continues to work efficiently.
Crop plants differ a great deal in their ability to survive and yield satisfactorily when grown in saline soils. Information on the relative tolerance of crops to a saline soil environment is of practical importance in planning cropping schedules for optimum returns. There are situations where farmers have to live with salinity problems, for example, in areas having saline water as the only source of water for irrigation. In other situations where good quality water is available for reclamation of saline soils, it is often helpful to grow crops simultaneously with reclamation efforts to make reclamation economic.
There is much literature on the relative tolerance of different crops to soil salinity obtained under a vast range of soil, climatic and salinity conditions. As will be discussed in section 3.3.2, tolerance to salinity is not a fixed property of a species but varies with the growth stage of the crop, climatic conditions and within the same species for different varieties of the crop. These factors render the task of evaluating crop salt tolerance data difficult. Also the methodologies adopted by different workers for studying tolerance have varied from water culture experiments on the one extreme to field studies with little control on the root zone salinity on the other. Maas and Hoffman (1977) and Maas (1984) compiled and reviewed available data to arrive at the best assessment of the relative salt tolerance of agricultural crops. The information presented by these workers has now been extensively quoted and used for practical purposes. Salt tolerance data for several crops, as presented by Maas and Hoffman are reproduced in Figure 8 (a to h). These figures show that, in general, crop yields were not reduced significantly until a threshold salinity level was exceeded, and then the yields decreased approximately linearly as the salinity increased beyond the threshold. The salt tolerance curve for each crop was obtained by calculating a linear regression equation for the yield beyond the threshold point. From the curves presented in these figures, (EC) relative yield (Y) (percent) at any given soil salinity can be calculated by the equation:
100 (EC o - EC e )
where EC 100 is
EC o - EC 100
the salinity threshold value (EC e where Y = 100) and EC the salinity at zero yield (EC e where Y = 0). Values of EC 100 and EC o for a given crop can be taken from the appropriate figure. Taking cotton as an example, EC 100 =8 dS/m and EC o =27.0 dS/m (Figure 8b). Therefore, the relative yield at an EC of say, 10 dS/m will be:
= 100 (27.0 - 10.0)/(27.0 - 8.00)
= 100 (17.0)/(19.0)
= 89 percent.
The shaded areas in Figures 8a to 8h indicate a qualitative salt tolerance rating for each crop. The five areas represent respectively, from left to right, sensitive, moderately sensitive, moderately tolerant, tolerant and unsuitable for crops.
It was emphasized by Maas and Hoffman (1977), Maas (1984) and we wish to repeat that the relative tolerance of crops as depicted in Figures 8a to 8h does not represent the absolute salt tolerances independent of other factors. Only a guide is furnished to the relative tolerance of crops. Whereas actual tolerance will vary with climate, cultural practices and other variables, relative tolerance should apply under most conditions.
Very often tolerance to saline and sodic conditions is not adequately differentiated and this can lead to inappropriate conclusions. The data in Figures 8a to 8h are for saline conditions and do not apply for sodic conditions. For example, barley is known to be a tolerant crop to saline conditions (Figure 8a) but is not a tolerant crop for sodic conditions. Similarly cotton, while tolerant of saline conditions, is only moderately so (or even sensitive at some growth stages) to sodic conditions. On the other hand, rice, though considered only moderately sensitive to saline conditions (Figure 8a), is highly tolerant of sodic conditions. Information on tolerance to sodic conditions is summarized in a subsequent section.
Although rice is not tolerant to excess salinity, it is a crop favoured in saline soils and, in fact, is preferred over other tolerant crops during the initial stages of reclamation of many saline soils. This is chiefly due to the system of lowland rice culture that is advantageous to the crop rather than to the tolerance of the crop to soil salinity. The system of lowland rice culture involving maintenance of standing water almost throughout the growing season brings about a significant reduction in the root zone salinity by leaching and dilution of the salts. Thus the crop is at no stage subjected to the salinity stress that might be indicated by the initial soil analysis. Rice is an important crop in many coastal regions and is grown during the rainy season. Although initially the soil salinity may be high, after one or two rains salinity is reduced in the upper few centimetres enabling planting of seedlings grown in a relatively good soil. Salinity is usually a greater constraint in the dry season when the evaporative demand is high and supply of good quality water restricted. Under these conditions when groundwater of high salinity must be used, salinity becomes a major constraint to obtaining satisfactory crop yields.
Reclamation requires that the soluble salts from the profile are leached and drained through a suitable system of drainage, but good quality water is often a major constraint in arid regions. Therefore, leaching alone for prolonged periods is not justifiable and so a rice crop is conveniently grown during reclamation. Rice gives satisfactory yields even when the electrical conductivity of the saturated soil extract is 20 to 25 dS/m in the upper layers (Van Alphen, 1975; Yadav and Girdhar, 1981). Even on soils with low infiltration rates the accumulated depth of water percolating through the soil profile in one rice season may be 100 to 200 mm. Table 12 gives data on salinity changes due to cropping with rice. Although leaching under continuously ponded conditions has the disadvantage of being less efficient for salt leaching compared to intermittent irrigation, the benefit of simultaneous crop production makes rice an ideal crop during reclamation of saline soils.
Table 12 CHANGES IN SOIL SALINITY DURING THE RECLAMATION OF A HIGHLY SALINE SOIL BY GROWING A RICE CROP (Van Alphen, 1975)
Soil
Depth cm
Initial
Before 1st rice crop
After 1st rice crop
After 2nd rice crop
After 3rd rice crop